Since antiquity, a multiplicity of geodetic methods and geodetic devices have been known for the purpose of measuring a target point. Here, distance and angle from a measuring device to the target point being measured adopted as spatial standard data and, in particular, the location of the measuring device together with any reference points present are acquired as said standard data.
A generally known example for such measuring devices and/or geodetic devices is provided by the tachymeter or by a total station, which is also denoted as an electronic tachymeter or computer tachymeter. Such a geodetic measuring device of the prior art is described, for example, in publication EP 1 686 350. Such devices have electrosensor angle and distance measurement functions that permit determination of direction and distance to and from a selected target. The angle and distance variables are determined in this case in the internal reference system of the device, and still have to be linked, if appropriate, to an external reference system for an absolute determination of position.
Modern total stations have microprocessors for further digital processing and storage of acquired measured data. As a rule, the devices are produced in a compact and integrated design, coaxial distance and angle measuring elements, as well as arithmetic logic, control and memory units are mostly integrated in one device. Means for motorizing the targeting optics, for reflector-less distance measurement, for automatic target seeking and tracking, and for remote control of the entire device are integrated, depending on the upgrade level of the total station. Total stations known from the prior art further have a radio data interface for establishing a radio link with external periphery components such as, for example, with a data acquisition device that can, in particular, be designed as a handheld data logger, field computer, notebook, minicomputer or PDA. By means of the data interface, it is possible for measured data acquired and stored by the total station to be output for external further processing, for externally acquired measured data to be read into the total station for the purpose of storing and/or further processing, for remote control signals to be input and/or output for the remote control of the total station or of a further external component, particularly in mobile use in the field, and for control software to be transcribed into the total station.
The measuring accuracy attainable in the measuring operation varies depending on the design of the target point to be measured. If, however, the target point is represented by a target reflector specifically designed for measurement—such as a panoramic prism—it is therefore possible to achieve substantially more accurate measurement results than given in a reflector-less measurement, for example in relation to a point to be measured on a house wall. The reason for this is, inter alia, that the cross section of the emitted optical measurement beam is not punctiform but two-dimensional, and therefore scattered measuring radiation is received not only at the target point actually to be measured, but also from points in the immediate surroundings of the field of view of the target point to which the measuring radiation is likewise applied. For example, the roughness of the surface of the point to be measured influences the accuracy of reflector-less measurements in a known way.
In addition, such geodetic devices mostly have a sighting device for sighting or aiming at target points. In a simple design variant, the sighting device is designed, for example, as a target telescope. Modern devices can, moreover, have a camera for acquiring an image that is integrated in the target telescope and is aligned, for example, coaxially or in parallel, it being possible for the acquired image to be displayed, in particular, as a live image on a display of the total station, and/or on a display of the peripheral device—such as a data logger—used for the remote control. The optics of the sighting device can in this case have a manual focus—for example, a setting screw for varying the focal position of the optics—or have an autofocus, the focal position being varied, for example, by servomotors. Automatic focusing devices for target telescopes of geodetic devices are known, for example, from DE 19710722, DE 19926706 or DE 19949580.
The optical system of the sighting device includes, in particular, an objective lens group, a focusing lens group and an eyepiece, which are arranged in this sequence starting from the object side. The position of the focusing lens group is set as a function of the object distance so as to result in a sharp object image on a reticule arranged in the focusing plane. Said object image can then be observed through the eyepiece, or recorded with the aid of a coaxially arranged camera.
In the case of a known sighting telescope with an autofocus system, directly after the AF start key has been pressed then the focusing lens group is moved from the respective position into another position in order to focus a sighted object.
With a phase difference detection system, a point first detected is regarded as the current focal point of the sighted object so that the autofocus system moves the focusing lens group into an axial position that corresponds to this focal point, whereupon the focusing lens group is stopped.
With such an autofocus control, there is a need to align the target telescope with the target before carrying out the autofocus process. In addition, when a target prism is automatically focused either the mount holding the prism, or an image reflected at the prism is focused. It follows that which of the two images (an image of the mount or an image of the telescope) is to be focused with the autofocus system is not determined reliably. In particular, the front side of the object lens of the sighting telescope can be seen as a dark image, while the contrast of the telescope housing is generally strong, and so the telescope is often deflectively focused on its own image reflected at the prism, and not focused on the prism.
By way of example, a total station is set up in terrain in the case of a typical one man measurement task with a target reflector. The user moves a handheld measuring rod, which carries the target reflector, onto a target point to be measured, whereupon the position of the target reflector, and thus of the target point, can be determined as follows. The total station is remotely controlled, in particular, by the user carrying the measuring rod, this being done by means of a data logger linked to the total station by radio. The data logger can in this case be fitted on the measuring rod equipped with the target reflector, or the user can, in addition, hold it in his hand next to the measuring rod.
The sighting of a target reflector can in this case be performed, in particular, by means of a live image displayed to the user in the data logger display and which is provided by a camera—arranged, for example, coaxially in the target telescope or with an alignment parallel to the target telescope—as sighting device of the total station. Consequently, by using the live image, the user can align the total station correspondingly with the desired target detectable in the live image.
However, if the live image is not focused on the target, but on another distance, it can often happen that the target in the live image is able to be detected and identified by the user only with difficulty. Such focusing that is wrong or unsuitable for detecting the target can result because the autofocus function automatically focuses on an object located in the center of the camera image. Before the target is detected and sighted, it is, however, generally located not in the center of the image, but in the periphery of the image, and this leads to the focusing that is unsuitable for the user. Consequently, of greater ease in detecting and identifying the target it has so far been possible for a complicated manual refocusing of the camera optics to be necessary for focusing on the target, so that the total station can thereupon be aligned with the target that can be detected in focus in the image.
If the geodetic measuring device has an automatic target search function in the case of which a large field of view region of the measuring device is scanned, for example by means of a rotating, vertically spread measurement beam, the target reflector can be found automatically in the field of view of the total station. However, problems can arise, particularly when identifying the target reflector, during such an automatic search. For example, it also happens in this case that false reflections which cannot be unambiguously distinguished from the reflection of the target reflector that is actually to be measured are recorded from further reflectors in use on a building site for measuring purposes, or else from further reflecting objects, such as automobile lights or glass panes, lying in the field of view region of the total station. To date, this has mostly required a user to identify one of the recorded reflections as that of the target reflector reflection in a complicated and reliable way.
It can also happen in the case of automatic target tracking of a target reflector that the total station loses the target reflector from the sight. Here, as well, an automatic target search can be carried out in order to find the target reflector again, an occurrence of the above described problems being possible in the identification of the target reflector from the set of reflecting objects recorded in the target search, which represent all target point candidates coming into consideration as target point.
As regards reflectorless measurements relating to sighted target points, further problems can arise in addition when further objects lying close to the actual sighted target object are located in the field of view of the measuring device. It can happen in this case when measuring radiation is also applied to the further objects, and that a portion of measuring radiation that is scattered on these objects is received. Admittedly, it is then possible in principle to determine the several distances from the respective components of the measuring radiation that are backscattered by the various objects, but it is necessary nevertheless to identify the distance from the target object actually to be measured, and to assign the distance sought to the target object. In the case of total stations of the prior art, such identification and assignment likewise mostly have to be carried out by a user, and are therefore complicated and subject to error.